The present application is directed toward the field of power rectification and specifically toward a field effect transistor (FET) shunt regulator for use with a permanent magnet alternator (PMA).
Machines for creating multiphase alternating current (AC) electrical power are well known in the art, as are methods for converting the AC electrical power into direct current (DC) electrical power for use with applications requiring DC power. Often when converting from AC to DC, a higher DC voltage is generated than can be handled by the DC load. When this occurs a shunt regulator is used to reduce the power seen by the load.
A shunt regulator operates by “shunting” a portion of the AC current to a neutral line. This short circuits out the rectifier portion during a portion of the period of the AC current. A typical shunt regulator will alternate between shunting and not shunting at a high enough frequency that a response time of a DC rectifier renders an approximately constant DC output power at the desired level.
One standard shunt regulator design utilized in the art is an FET shunt. An FET shunt uses FET's to create a short circuit from a phase voltage line connected to the source node of the FET to a neutral line connected to the drain node of the FET. The short circuit is created when the FET is turned on via a control signal thereby connecting the source and drain nodes in a virtually unimpeded manner.
When an FET shunt such as the one described above is utilized with a PMA there is necessarily a return current that must return to the PMA in order to form a complete circuit. While the FET shunt is on (aka shunting) the connection between the source and drain provides unimpeded access across the FET for return current from the neutral line. However, when the FET shunt is off there is no connection between the source and drain and the current must return through a different path. In a typical design in the art the current will return across a body-drain connection in the FET. The connection is referred to as a body-drain diode. The body-drain diode connection acts in a similar manner as a diode and typically has a voltage drop of around 1.4V across it. This voltage drop causes power dissipation within the FET resulting in a lower efficiency for the shunt regulator as well as reducing the lifespan of the FET itself.